This invention relates to methods and apparatus for strategically strengthening steel members that can exhibit complex 3D geometries including, for example, vehicular components such as impact door beams, roof and pillar support components and suspension components.
Automotive vehicles employ crash energy management impact beams for protection of passengers, many of which utilize a closed structure type, e.g., square or cylindrical tubing. Such closed beams typically have both a generally uniform wall thickness and lack exposed edges or flanges, the absence of which renders them better suited for conventional induction heat treatment. In such heat treatment processes, the beam is surrounded by a corresponding heat treatment induction coil that, when energized, heats the beam to a target temperature range after which the heated beam is quenched to fix the desired structural properties.
Alternatives to the tubular beam structures may be prepared by taking flat steel sheets and then forming an open beam structure by cold stamping, rolling and/or milling. The hardness and strength of such open beam structures, however, can be limited by the requirement that the metal hardness must be low enough so as to allow for the degree of deformation that will be induced by the stamping and/or rolling operations. As another alternative, harder steel may be hot formed into the desired configuration, but the added complexity of such processing results in higher costs per piece and in the initial capital investment.
Previous efforts to form open section type impact structures from low strength steel, and then heat treat the structures using conventional post-forming techniques have several limitations that tend to produce unacceptable results. For example, using standard induction heating with encircling coils, the free or exposed edges of the structures tend to become overheated and/or burned, while the remainder of the structure may fail to reach the target temperature range. As a result, the heat treated structure will not tend to 1) exhibit the target strength characteristics and/or 2) exhibit sufficiently uniform characteristics. Similarly, the heated open section type impact structures are more prone to distortion during subsequent quenching operations. Accordingly, it has been difficult to meet the required quality standards for cost-effective production of open section type impact structures for assembly in an automobile or other vehicle.
As used herein, “rolling” refers to a metal forming process in which metal stock is passed through at least one pair of rollers and is classified according to the temperature of the metal being rolled. If the temperature of the metal is above its recrystallization temperature while it is being worked, e.g., rolled, drawn or stamped, then the process is considered to be “hot” working, but if the temperature of the metal is below its recrystallization temperature, the process is considered to be “cold” working.
Steel hardening processes include thermal processes during which the steel is heated to a sufficiently high temperature (by, for example, direct application of a flame, contact heating or induction heating) then cooled rapidly, i.e., quenched, to fix the desired microstructure. In a “case” hardening process the necessary temperatures are generally confined to a surface portion of a steel element to form a “case” of hardened material, e.g., martensite, over a core of softer steel. Steel hardened in such a fashion tends to exhibit a combination of toughness and durability to provide a long service life while resisting catastrophic failure.
Other processes may be utilized for case hardening steel including, for example, carburization in which the steel is exposed to a carbon rich environment while being maintained at elevated temperatures and then quenched to provide a carbon-enriched surface layer. Depending on the configuration and intended use of the steel element, carburization can be applied only to selected regions to provide, for example, increased durability for load bearing surfaces.
Nitriding is another hardening technique in which the surface of the steel is exposed to a nitrogen rich environment while being maintained at elevated temperatures and then quenched to provide a nitride-enriched surface layer. The nitrogen reacts with one or more nitride forming elements including, for example, chromium, molybdenum and/or aluminum, present in the steel alloy. Nitriding can provide certain advantages as a hardening technique including, for example, reduced distortion that makes it suitable for hardening structural elements after they have been quenched, tempered and machined into substantially final form. Other case hardening techniques well known to those skilled in the art include, for example, cyaniding, carbonitriding and ferritic nitrocarburizing.
Differential hardening is a method used for providing a local increase in the hardness, e.g., the cutting edge of a tool, without rendering the bulk of the tool unacceptably brittle. In order to achieve this effect, the tool may be subjected to differential cooling in which the cutting edge is cooled more rapidly than the bulk of the tool by insulating or otherwise protecting the bulk of the tool from the quenching operation. A related process, differential tempering relies on quenching the tool or piece uniformly, then differentially tempering one or more portions with a targeted heat source to reduce the hardness in the heated portions.